Someone who wines and dines girls in the attempt of sleeping with them but frequently comes up short.-only to describe the events as an almost success.
Phil: hey how'd things go with christy last night?
Sean: I almost fucked her.
Phil: ahh you suck, youre such an almoster.
Someone who "almost" did something, didn't, but still wants to soundbad ass for almost doing it.
"Yea, I almost joined the Marines, but I knew I would punch out a Drill Instructor if they yelled at me. I'm crazy man, I'll just snap, too crazy for the Military, they couldn't handle me."